In the mobile communication, there is a case where a signal level is fluctuated instantaneously on a receiver side due to multipath fading or the like, and where a reception quality of an uplink signal at a base station or a reception quality of a downlink signal at a mobile station is therefore significantly deteriorated. A reception diversity and a transmission diversity are techniques for reducing such deterioration in the reception quality.
The reception diversity is a technique to receive a signal by use of a plurality of antennas, on a receiver side. The transmission diversity is a technique to transmit a plurality of signals having different symbol patterns or amplitudes by use of a plurality of antennas, on a transmitter side.
The transmission diversity can attempt to reduce fluctuation of a signal level without increasing the scale of circuits or the number of antennas on the receiver side, and is therefore applied mainly to signal transmission in a downlink from a base station to a mobile station (see Non-Patent Document “3GPP RAN TS25.214 V6.2.0.”, June 2004, for example).
The transmission diversity is broadly categorized into two schemes, an open loop transmission diversity scheme and a closed loop transmission diversity scheme.
In the closed loop transmission diversity scheme, a mobile station transmits a command for instructing a phase of a transmission signal to a base station through an uplink, in order to obtain a high reception power by a signal combining at the mobile station. Since there may be a case where an error occurs in the course of transmission through the uplink, the mobile station makes a judgment on the phase of the transmission signal actually set by the base station. This judgment is called an antenna verification (see Non-Patent Document “3GPP 25.214 V5.8.0, Annex A”, April 2004).
The antenna verification is executed by use of at least one of a predicted value (a prior probability) of a command error rate and an error rate (a posterior probability) figured out by use of a reception signal of a pilot signal contained in a dedicated channel.
However, an application of the transmission diversity has not always contributed to improvement in the reception quality. By contraries, there has also been a case where an application of the transmission diversity has led to deterioration in the reception quality as compared to a case of not applying the transmission diversity.
For example, it is possible to obtain an effect of a time diversity (hereinafter referred to as a “user diversity effect”) by allowing the base station to perform a high-speed scheduling by use of a downlink shared channel. However, there has been a case where applying the open loop transmission diversity scheme reduced a user diversity effect, and where therefore not applying this scheme has resulted in a higher user diversity effect.
FIG. 1 shows a user diversity effect in the case where a base station not applying the transmission diversity transmits signals by use of a single antenna. FIG. 2 shows a user diversity effect in the case of using the open loop transmission diversity scheme. In FIG. 1 and FIG. 2, the vertical axis indicates a reception power and a downlink transmission rate while the horizontal axis indicates time.
As shown in FIG. 1 and FIG. 2, the reception powers show sharper increases and decreases in the case of not applying the transmission diversity (FIG. 1), and therefore a higher user diversity effect than in the case of applying the transmission diversity (FIG. 2) can be achieved by allocating a shared channel to a user having a high reception power. As a result, it is possible to enhance a downlink transmission rate and thereby to increase throughput.
Meanwhile, in the closed loop transmission diversity scheme, it has been sometimes difficult to render errors of the antenna verification sufficiently low, when the dedicated channel had a low speed, and when the power was small, for example. In particular, when a mobile station performs a soft handover, the reception power of each base station connected to the mobile station is reduced by performing a selective combining or the like. In short, the reception power per base station is reduced. For this reason, the reception power of commands is reduced while an error rate of the commands is increased, thereby increasing the probability of occurrence of errors in the antenna verification. As a result, there has been the case of causing not only reduction in the transmission diversity effect but also deterioration in the reception quality as compared to the case of not applying the transmission diversity.
As described above, a disadvantage caused by the transmission diversity has exceeded an advantage obtained by the transmission diversity in some cases. As a consequence, an application of the transmission diversity has sometimes led to deterioration in the quality of the downlink, reduction in the radio capacity attributable to an increase in required transmission power, and deterioration in the transmission efficiency in comparison with the case of not applying the transmission diversity.
In this case, it is conceivable to perform the transmission from a single antenna without applying the transmission diversity. However, the base station designed to be capable of applying the transmission diversity is equipped with amplifiers for the respective antennas in order to transmit signals from the plurality of antennas. The maximum power of each amplifier is equivalent to an electric power obtained by dividing the maximum power which can be outputted from the amplifiers of the base station not applying the transmission diversity by the number of the amplifiers.
For example, while a base station not applying the transmission diversity includes an amplifier having the maximum output of 20 W, a base station applying the transmission diversity includes two amplifiers each having the maximum output of 10 W.
For this reason, the transmission power is reduced when the base station capable of applying the transmission diversity performs the transmission by use of the single antenna. This results in reduction in the radio capacity and deterioration in the transmission efficiency. Eventually, a radio area that can be actually covered by the base station is reduced.